High pulse repetition rate gas discharge laser

ABSTRACT

A pulsed gas discharge laser operating at an output laser pulse repetition rate of greater than 4 kHz and a method of operating same is disclosed which may comprise a high voltage electrode having a longitudinal extent; a main insulator electrically insulating the high voltage electrode from a grounded gas discharge chamber; a preionizer longitudinally extending along at least a portion of the longitudinal extent of the high voltage electrode; a preionization shim integral with the electrode extending toward the preionizer. The preionizer may be formed integrally with the main insulator. The preionization shim may substantially cover the gap between the electrode and the preionizer. The apparatus and method may comprise an aerodynamic fairing attached to the high voltage electrode to present an aerodynamically smooth surface to the gas flow.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a continuation application of U.S. patentapplication Ser. No. 11/169,203, entitled HIGH PULSE REPETITION RATE GASDISCHARGE LASER, filed on Jun. 27, 2005, Attorney Docket No.2005-0094-01, the contents of which are hereby incorporated byreference.

The present application is related to U.S. patent application Ser. No.10/877,737, entitled HALOGEN GAS DISCHARGE LASER ELECTRODES, filed onJun. 25, 2004, Attorney Docket No. 2004-0033-01, and of Ser. No.10/815,387, entitled GAS DISCHARGE LASER CHAMBER IMPROVEMENTS, filed onMar. 31, 2004, Attorney Docket No. 2003-0092-01, the disclosures ofwhich are hereby incorporated by reference.

FIELD OF THE INVENTION

The present invention is related to high pulse repetition rate, e.g., 4kHz and above gas discharge laser systems, e.g., excimer and molecularfluorine laser or CO₂ systems.

BACKGROUND OF THE INVENTION

Gas discharge lasers, e.g., KrF, ArF, F₂, XeF, XeCl, CO₂ and the likelaser systems operating at pulse repetition rates of up to 6 kHz andabove present certain operational problems, related, in part, todisturbances to the gas flow circulating around the lasing mediumcontaining chamber, and particularly in the vicinity of the highrepetition rate gas discharges occurring between elongated gas dischargeelectrodes as such electrodes are well known in the art. Applicantspropose certain improvements and modifications to existing assembliescomprising, e.g., electrodes, a preionizer(s), a main insulator(s) andelectrode supports.

SUMMARY OF THE INVENTION

A pulsed gas discharge laser operating at an output laser pulserepetition rate of greater than 4 kHz and a method of operating same isdisclosed which may comprise a high voltage electrode having alongitudinal extent; a main insulator electrically insulating the highvoltage electrode from a grounded gas discharge chamber; a preionizerlongitudinally extending along at least a portion of the longitudinalextent of the high voltage electrode; a preionization shim integral withthe electrode extending toward the preionizer. The preionizer may beformed integrally with the main insulator. The preionization shim maysubstantially cover the gap between the electrode and the preionizer.The apparatus and method may further comprise a high voltage electrode;a main insulator insulating the high voltage electrode from a groundedgas discharge chamber; the high voltage electrode being disposed in anelectrode receiving pocket in the main insulator and formed to presentan elongated discharge receiving area facing another electrode withinthe gas discharge chamber, with adjacent side surfaces of the highvoltage electrode slanting away from an outer extent of side walls ofthe pocket in the longitudinal extent of the pocket and the high voltageelectrode, forming a gas flow disturbance pocket; an aerodynamic fairingattached to the high voltage electrode and substantially closing the gasflow disturbance pocket and presenting an aerodynamically smooth surfaceto the gas flow. The main insulator may be formed to present anaerodynamically smooth face in the direction of the gas flow and thefairing aerodynamically smooth surface being generally aligned with themain insulator aerodynamically smooth surface. The aerodynamic fairingmay be on one side of the elongated discharge receiving area and apreionizer may be positioned on the other side of the elongateddischarge receiving area. The one side is the downstream side or theupstream side. The aerodynamic fairing may comprise a pair ofaerodynamic fairings one on each side of the elongated dischargereceiving area. The apparatus and method may comprise an aerodynamicblock presenting an extension of the aerodynamically smooth surface ofthe main insulator. The high voltage electrode may be formed with anintegral shim extending toward the preionizer and presenting anaerodynamically smooth surface to the gas flow. The apparatus and methodmay further comprise an elongated electrode support bar formed to havean elongated dugout portion; an elongated electrode attached to theelectrode support bar in the elongated dugout portion; an upstreamfairing positioned in the elongated dugout portion presenting with theelectrode support bar and electrode an aerodynamically smooth surface toa gas flow past the electrode. The apparatus and method may furthercomprise a downstream fairing positioned in the elongated dugout portionpresenting with the electrode support bar and the electrode anaerodynamically smooth surface to a gas flow past the electrode.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a cross-sectional view orthogonal to a longitudinalextension of electrodes in a gas discharge laser according to aspects ofan embodiment of the present invention according to which ideal oroptimized gas flow through the gas discharge region between theelectrodes of such a gas discharge laser were tested;

FIG. 2 shows the cross-sectional view of the apparatus of FIG. 1 withgas discharge electrical components formed to closely approximate theideal or optimized flow conditions according to aspects of an embodimentof the present invention;

FIGS. 3 and 4 show cross sectional views of a shimmed electrodeaccording to aspects of an embodiment of the present invention;

FIG. 5 shows a perspective orthogonal view of a portion of a shimmedelectrode according to aspects of an embodiment of the presentinvention;

FIGS. 6 and 7 show a side view of an electrode fairing according toaspects of an embodiment of the present invention;

FIG. 8 shows a cross-sectional view of the electrode fairing of FIGS. 6and 7 at cross-sectional lines 8-8 in FIG. 6;

FIGS. 9-11 show cross-sectional views at different points along theelongated extent of the electrodes of a gas discharge laser in which theother of the electrodes and its mounting and fairings are formedaccording to aspects of an embodiment of the present invention for moreoptimized or idealized aerodynamic gas flow performance;

FIGS. 12 and 13 show aspects of embodiments of the present invention inwhich an electrode has a pair of fairings on either side of a dischargereceiving portion of the electrode.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

In order to allow for higher repetition rate operation of e.g., gasdischarge lasers, e.g., excimer lasers, such as ArF, KrF, XeF, and XeCllasers, or F₂ lasers or CO₂ lasers or the like, e.g., at laser outputlight pulse repetition rates of up to and exceeding 6 kHz, applicantshad expected that the blower motor speed would have to increase about50% above those needed for currently used lasers of such a type ataround 4 kHz pulse repetition rates. This can create a need fortremendously higher blower power, increasing on the order of as the cubeof the speed increase. According to aspects of embodiments of thepresent invention applicants propose certain improvements to the flowpath of the laser gas being moved by the blower motor within the laserchamber in order to reach the required gas flow amounts (basically gasflow speed, although those skilled in the art will realize that the flowspeed is not a constant either in the elongated longitudinal electrodeaxis direction or a direction perpendicular to that direction, due,e.g., to flow restriction of surrounding components, flow resistance atthe boundaries of the flow region, etc.).

In the past such parameters as arc free blower speed (“AFBS”) have beenmeasured to determine, e.g., the blower speed needed. The blower speednecessary is determined to be able to move sufficient gas away from thedischarge region between the elongated electrodes in such devices. Thisis necessary to on the one hand assure the next sequential discharge isa good one (maintains the same laser output light energy for a givenapplied voltage through the discharge formed lasing medium) because thegas through which the discharge occurs to form the lasing medium is“fresh”. On the other hand the gas flow sufficiency is determined bybeing able to avoid arcing between one of the electrodes and thegrounded chamber walls or between electrodes through ions present in thegas through which the prior discharge occurred between the electrodes.If such ions remain close enough to the electrodes to form an electricalpath to the chamber wall at the time when the high voltage one of theelectrodes is raised in voltage to ultimately cause the dischargethrough the lasing medium laser gas such arcing can occur. The lattereffect also, to among other things, results in instability of the laseroutput light pulse energy from pulse to pulse.

FIG. 1 shows a cross-sectional view of a test fixture that applicantsused to test the gas flow properties of an “idealized” flow path for thegas through the discharge region in the gap 40 between electrodes, whichare, along with surrounding elements simulated by a cathode blank 14 andan anode blank 16. The cathode blank 14 simulates an existing cathodeand adjacent portions of a main insulator 20 (shown in FIG. 2) withoutgaps that have traditionally been present in such assemblies, e.g., asshown in FIG. 9 around the cathode 36. In addition flow smoothing blocks44 were inserted to fill in a region on either side of the lateralextent of the main insulator 20, which openings are also shown in FIG. 9in the vicinity of either side of the main insulator 20. The flowsmoothing blocks 44 contain a plurality of narrow slits 38 that allowfor the current returns to pass through the smoothing blocks 44.

Applicants found that this “idealized” gas flow path including anessentially uniform upper inlet gas flow surface 45 and upper exit gasflow surface 46 in combination with respective opposite lower inlet gasflow surface 47, which may correspond to an arcuate shaped surface,e.g., formed by an anode fairing or a portion of an anode support bar 42and an exit gas flow surface formed, e.g., by an downstream anodefairing, e.g., as shown in FIGS. 12 and 13 or by a portion of the anodesupport bar itself, as shown, e.g., in FIGS. 9-11 and discussed below.

Applicants found that measured gas flow speeds were indicative of thefact that the necessary blower speed with such an aerodynamically“idealized” gas flow path for, e.g., an increase in pulse repetitionrate by 50% from 4 kHz to 6 kHz, was achievable and not the expected 50%increase in blower speed with the concomitant increase in requiredblower power supply. The idealized smooth upper and lower surfacesremove, e.g., the effects of turbulence and flow separation created,e.g., by gaps between components of the laser system in the vicinity ofthe discharge region 40 and/or otherwise in the gas flow path from theblower 18 to the portion of the lasing chamber 12 beyond the downstreamextent of the anode support bar 42. They reduce the amount ofturbulence, flow separation, eddies, and the like in the gas flowthrough the discharge region 40.

As can be seen from the illustrative partly schematic view in FIG. 2according to aspects of an embodiment of the present invention, withinthe requirements for using separate parts, e.g., the cathode 10 and maininsulator 20, the preionizer 25, current returns 60, 62, and the like,due, e.g., to their electrical, manufacturing, mechanical and structuralproperties and requirements, applicants propose according to variousaspects of the present invention to provide a more aerodynamic gas flowpath from the blower 18, through the gas discharge region 40 and pastthe anode and its fairings into the portion of the chamber 12 beyond theanode support bar 42.

According to aspects of embodiments of the present invention this mayinclude an integral cathode shim 10 and the cathode fairing 50. A maininsulator 20 may have a through hole 22 for a ground rod for integralpreionizer (“PI”) 30 operation. The integral shim 32 on the cathode 10may be specifically shaped, e.g., not only to provide the pre-ionizationfunction of bringing the high voltage on the cathode to the vicinity ofthe preionizer 25 ground rod 30, but also to shape/steer the gas flowefficiently toward the discharge region 40 between the electrode gap. Inthis manner, the traditional gap between the cathode 36, e.g., as shownin FIG. 9, and main insulator 20 and preionizer (where the preionizer 25is an upstream preionizer 25, e.g., as shown in FIGS. 9 and 10, andproviding a surface exposed to the passing gas flow that is more likethe aerodynamically smooth portion of the surface 45 in the “idealized”gas flow path of FIG. 1.

Since downstream flow separation on the anode assembly has been shown toincrease the arc-free blower speed, according to aspects of anembodiment of the present invention a commensurate downstream/fastbackfairing 51 (as shown in FIGS. 12 and 13 on the anode 42 may be matchedwith as a corresponding downstream/fastback fairing 50 for the cathode10. The cathode fairing 50, as is the anode fairing 51, may be made ofan insulator, e.g., of the same ceramic material as is the maininsulator 20.

Previously, this might have been implemented by complex machining of thecathode and fairing and bolting them together to retain the cathode'soriginal shape. According to aspects of an embodiment of the presentinvention the cathode fairing 50 may be, e.g., a ceramic part, in theshape of a wedge 50, e.g., as shown in FIGS. 6 and 7. This wedge 50 withtwo integral mounting pads 52, 54 outboard of the cathode 10 (in theelongated direction) may then be affixed to the main insulator 20 or tothe upper chamber half 76, e.g., with ceramic screws or metal screwslike the anode fairing 51. Alternatively, a rounded portion 78 of thecathode, as illustrated, e.g., in FIG. 5 with respect to the shimmedcathode 10, may be partially machined away and the mounting pads 52, 54attached there with metal or ceramic screws.

In addition, the gas flow blocks 38 and current return attachment platefairings which were part of the testing fixture of FIG. 1 may also beincluded in the embodiment of FIG. 2 to further give the effect of the“idealized” gas flow passage across the upper inlet and exit surfaces ofthe gas flow path past the electrodes, in order to make them moreclosely approach the aerodynamically smooth surfaces of the “idealized”gas flow passage of FIG. 1.

As shown in FIGS. 12 and 13, the cathode fairing may be used on bothsides of the cathode 36, with the wedge shaped fairings 100, 102 similarto the one shown in FIG. 2, as shown in FIG. 12, and more rectangularlyshaped fairings 104, 106 shown in FIG. 13. These fairings 104, 106, asillustrated, may, e.g., have noses 110 shaped to more conform to roundededges of, e.g., the preionizer 25 on the upstream or downstream side, ifany, or the main insulator 20 on either the upstream side or downstreamside, or both, as applicable, and with flat protrusions 112 extendingtoward the discharge receiving portion of the cathode 36′.

According to aspects of an embodiment of the present invention the gasflow in the vicinity of the discharge region 40 may be further improvedby approaching an idealized flow region while maintaining the necessarydistinct nature of certain parts, e.g., because they are made ofdifferent materials or are not easily fabricated as a single piece, etc.The effects of turbulence and aerodynamic drag and eddy formingprotrusions in the neighborhood of the discharge region 40, among otherthings which impact the gas flow properties, and, therefore, also theAFBS for a given configuration and repetition rate, can be significantlyameliorated on the side of the other electrode, as well as the side ofthe cathode as noted above.

In the past, the anode support bar in lasers sold by applicants'assignee, Cymer, Inc., e.g., models 6XXX and 7XXX laser systems, havehad an anode support bar 54 with a flat upper surface, such asillustrated in FIGS. 1, 2, 12 and 13, to which have been attached ananode 34 and anode fairings 51, 53. The flow shaping fairings 51, 53 andanode 34 itself, extending above the anode support bar 42, have beenshown to also provide locations for drag, turbulence and flowseparation, etc.

These can be substantially eliminated by, e.g., providing an anodesupport bar 42 such as illustrated in FIGS. 9, 10 and 11, wherein, e.g.,the upper surface is not flat, but is shaped for flow aerodynamics, andcontains, e.g., a dugout region 80 into which may be inserted anode 34and upstream fairing 90 and downstream fairing 92. In this manner, theentire anode assembly can be made to present a more seamless aerodynamicflow profile to the gas flow through the discharge region 40. The frontportion 72 of the anode support bar 42 forming a cutoff for the blowerfan 18 may be made of separate piece as a manufacturing convenience toallow for the attachment of the upstream current returns attached to anattachment strip 70 that is secured between the front portion piece 72and the anode support bar 42 with the front cutoff portion 72 attachedto the anode support bar by screws 74. However, for purposes of thepresent application, according to aspects of embodiments of the presentinvention it may be considered part of the anode support bar 42.

The downstream current return tines 62 may be fastened to the anodesupport bar 42 by a strip 64 held in place by a clamp plate 66 attachedto the anode support bar 42 by screws 68. The fairings 90, 92 may beconnected to the anode support bar by screws 86, with the upstreamfairing having a brass washer 82 and the downstream fairing having acrescent washer 84. The washers may be alternated from side to sidealong the length of the electrode fairings 90, 92.

To allow for higher repetition rate laser operation without the need fortremendously higher blower power, applicants also propose to replace ablade/dielectric electrode, e.g., an anode as originally designed andnow implemented in certain laser products of the applicants assignee,Cymer, Inc., e.g., models 6X10 and 7000, with the anode extending inheight above the adjacent fairings. This was done at least in part toensure a properly attached discharge even after the anode had worn downafter some use. But at higher rep rates up to and above 6 kHz applicantshave discovered a flow penalty is paid for this protrusion into the gasflow. Applicants have also discovered that reefing electrodes thatdevelop a natural reef from the reaction of lasing gas, e.g., fluorinewith material in the electrode with exposure to the lasing medium in thelasing gas caused by high voltage discharge between the electrodes inthe laser system or which are provided with an artificial reef there isvery little or non-existent wear over electrode life. Therefore,according to aspects of an embodiment of the present inventionapplicants propose to, e.g., have the electrode, e.g., the anode heightmatched with the fairing height, or perhaps even slightly lower than thefairing height to allow for the reef to grow where the electrode and thetype of laser system naturally grow reef coatings to allow the reefcoating to extend essentially to the fairing height. For example, asillustrated in FIGS. 9-11 the anode can be “even with” the adjacentfairing(s) when it is, e.g., within about ±0.5 μm from the highestextension of the respective fairing(s), which may also be true for otherembodiments with non-reefing electrodes being “even with” the fairingheight(s).

It will be understood by those skilled in the art that the aspects ofembodiments of the present invention disclosed above are intended to bepreferred embodiments only and not to limit the disclosure of thepresent invention(s) in any way and particularly not to a specificpreferred embodiment alone. Many changes and modification can be made tothe disclosed aspects of embodiments of the disclosed invention(s) thatwill be understood and appreciated by those skilled in the art. Theappended claims are intended in scope and meaning to cover not only thedisclosed aspects of embodiments of the present invention(s) but alsosuch equivalents and other modifications and changes that would beapparent to those skilled in the art. In addition to changes andmodifications to the disclosed and claimed aspects of embodiments of thepresent invention(s) noted above the others could be implemented.

While the particular aspects of embodiment(s) of the HIGH PULSE

REPETITION RATE GAS DISCHARGE LASER described and illustrated in thispatent application in the detail required to satisfy 35 U.S.C. §112 arefully capable of attaining any above-described purposes for, problems tobe solved by or any other reasons for or objects of the aspects of anembodiment(s) above described, it is to be understood by those skilledin the art that the presently described aspects of the describedembodiment(s) of the present invention are merely exemplary,illustrative and representative of the subject matter which is broadlycontemplated by the present invention. The scope of the presentlydescribed and claimed aspects of embodiments fully encompasses otherembodiments which may now be or may become obvious to those skilled inthe art based on the teachings of the Specification. The scope of thepresent HIGH PULSE REPETITION RATE GAS DISCHARGE LASER is solely andcompletely limited by only the appended claims and nothing beyond therecitations of the appended claims. Reference to an element in suchclaims in the singular is not intended to mean nor shall it mean ininterpreting such claim element “one and only one” unless explicitly sostated, but rather “one or more”. All structural and functionalequivalents to any of the elements of the above-described aspects of anembodiment(s) that are known or later come to be known to those ofordinary skill in the art are expressly incorporated herein by referenceand are intended to be encompassed by the present claims. Any term usedin the specification and/or in the claims and expressly given a meaningin the Specification and/or claims in the present application shall havethat meaning, regardless of any dictionary or other commonly usedmeaning for such a term. It is not intended or necessary for a device ormethod discussed in the Specification as any aspect of an embodiment toaddress each and every problem sought to be solved by the aspects ofembodiments disclosed in this application, for it to be encompassed bythe present claims. No element, component, or method step in the presentdisclosure is intended to be dedicated to the public regardless ofwhether the element, component, or method step is explicitly recited inthe claims. No claim element in the appended claims is to be construedunder the provisions of 35 U.S.C. §112, sixth paragraph, unless theelement is expressly recited using the phrase “means for” or, in thecase of a method claim, the element is recited as a “step” instead of an“act”.

1. A pulsed gas discharge laser operating at an output laser outputlight pulse repetition rate of greater than 4 kHz comprising: a highvoltage electrode; a main insulator insulating the high voltageelectrode from a grounded gas discharge chamber; the high voltageelectrode being disposed in an electrode pocket in the main insulatorand formed to present an elongated discharge receiving area facinganother electrode within the gas discharge chamber, with adjacent sidesurfaces of the high voltage electrode slanting away from an outerextent of side walls of the pocket in the longitudinal extent of thepocket and the high voltage electrode, forming a gas flow disturbancepocket; an aerodynamic fairing attached to the high voltage electrodeand substantially closing the gas flow disturbance pocket and presentingan aerodynamically smooth surface to the gas flow.
 2. The apparatus ofclaim 1 further comprising: the main insulator being formed to presentan aerodynamically smooth face in the direction of the gas flow and thefairing aerodynamically smooth surface being generally aligned with themain insulator aerodynamically smooth surface.
 3. The apparatus of claim1 further comprising: the aerodynamic fairing is on one side of theelongated discharge receiving area and a preionizer is positioned on theother side of the elongated discharge receiving area.
 4. The apparatusof claim 2 further comprising: the aerodynamic fairing is on one side ofthe elongated discharge receiving area and a preionizer is positioned onthe other side of the elongated discharge receiving area.
 5. Theapparatus of claim 3 further comprising: the one side is the downstreamside.
 6. The apparatus of claim 4 further comprising: the one side isthe downstream side.
 7. The apparatus of claim 3 further comprising: theone side is the upstream side.
 8. The apparatus of claim 4 furthercomprising: the one side is the upstream side.
 9. The apparatus of claim5 further comprising: the aerodynamic fairing comprises a pair ofaerodynamic fairings one on each side of the elongated dischargereceiving area.
 10. The apparatus of claim 6 further comprising: theaerodynamic fairing comprises a pair of aerodynamic fairings one on eachside of the elongated discharge receiving area.
 11. The apparatus ofclaim 2 further comprising: an aerodynamic block presenting an extensionof the aerodynamically smooth surface of the main insulator.
 12. Theapparatus of claim 4 further comprising: an aerodynamic block presentingan extension of the aerodynamically smooth surface of the maininsulator.
 13. The apparatus of claim 6 further comprising: anaerodynamic block presenting an extension of the aerodynamically smoothsurface of the main insulator.
 14. The apparatus of claim 8 furthercomprising: an aerodynamic block presenting an extension of theaerodynamically smooth surface of the main insulator.
 15. The apparatusof claim 3 further comprising: the high voltage electrode is formed withan integral shim extending toward the preionizer and presenting anaerodynamically smooth surface to the gas flow.
 16. The apparatus ofclaim 4 farther comprising: the high voltage electrode is formed with anintegral shim extending toward the preionizer and presenting anaerodynamically smooth surface to the gas flow.